The present invention relates to a pneumatic tire having an improved bead structure capable of improving the durability.
In general, heavy duty tires for trucks, buses and the like are provided with big bead portions to withstand heavy loads.
In Japanese Patent 724291, however, in order to improve bead durability, a pneumatic tire having relatively small bead portions was proposed. In this proposition, the rubber on the axially inside of the carcass shown in FIG. 9 by broken line is reduced in the volume. And on the radially outside of a bead core (c), a turnup portion (d1) and main portion (d2) of a carcass ply adjoin each other to extend parallel with each other.
In order to further improve durability of this bead structure, the present inventors studied mechanism of possible bead damage, and it was found that
a stress on the radially outer end (e) of the turnup portion (d1) increases as it approaches the maximum tire section width position, and
the stress is liable to cause small cracks (initial cracks) around the outer end (e), and
the small cracks grow radially inwards through between the turnup portion (d1) and main portion (d2) to cause separation failure.
It is therefore, an object of the present invention to provide a pneumatic tire, especially a heavy duty radial tire, in which the above-mentioned small cracks around the radially outer ends of the carcass ply turnup portions are prevented.
According to the present invention, a pneumatic tire comprises a tread portion, a pair of sidewall portions, a pair of bead portions each with a bead core therein, and at least one pair of an axially outer cord layer and an axially inner cord layer each extending radially outwardly from each bead portion, wherein the axially outer cord layer is terminated at a radial height not more than 50% of the section height of the tire, and the axially inner cord layer extends radially outwardly beyond the radially outer end of the axially outer cord layer, and a cord spacing between the axially outer cord layer and the axially inner cord layer decreases radially inwardly from the radially outer end of the axially outer cord layer.